The number of patients awaiting organs for transplantation continues to increase each year, and growing numbers of patients with organ failure die because no organs are available. Xenotransplantation offers a potentially unlimited supply of transplantable organs to meet this clinical need, but no effective regimens yet exist that will safely prevent hyperacute rejection of discordant xenografts. As the first portion of the Physician Scientist Award, the major objective of the proposed phase I research project is for the candidate to receive basic science training and a Ph.D. degree in the field of Immunology at Duke University. As part of the thesis project, the applicant will investigate the mechanism of complement and antibody-mediated hyperacute xenograft rejection in the guinea pig-to-Lewis rat model of discordant xenotransplantation. The first goal will be to establish whether hyperacute rejection is modulated in the neonatally B cell depleted Lewis rat. Next, adult rats will be depleted of B cells using in vivo administration of anti-B cell monoclonal antibodies. Peripheral lymphocyte subsets and xenoreactivity against guinea pig lymphocyte targets will be assessed by flow cytometry throughout these studies. Should these techniques fail to deplete NAbs, plasma exchange or xenogeneic organ perfusion will be utilized in an effort to reduce NAb levels and prolong guinea pig cardiac xenograft survival. The second goal is to evaluate the mechanism of complement activation during hyperacute xenograft rejection. Hemolytic and other assays will be utilized to dissect the nature of complement activation, and in particular the role of C3a and C5a in this process. These peptides will be purified and used to produce rat C3a and C5a specific mouse monoclonal antibodies. These monoclonals will then be used in an attempt to modulate C3a and C5a mediated damage in vivo. Using the mechanistic data derived from these studies, complement inhibitors specific for important steps identified in the hyperacute rejection response will be tested in vivo to prolong guinea pig-toLewis rat cardiac xenograft model. If indicated by these initial studies, therapy directed against both the complement and NAb components of the response will be combined in an effort to further prolong xenograft survival.